David Rosenstrauch created MAPREDUCE-5402:
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Summary: DynamicInputFormat should allow overriding of
MAX_CHUNKS_TOLERABLE
Key: MAPREDUCE-5402
URL: https://issues.apache.org/jira/browse/MAPREDUCE-5402
Project: Hadoop Map/Reduce
Issue Type: Improvement
Components: distcp, mrv2
Reporter: David Rosenstrauch
In MAPREDUCE-2765, which provided the design spec for DistCpV2, the author
describes the implementation of DynamicInputFormat, with one of the main
motivations cited being to reduce the chance of long-tails where a few leftover
mappers run much longer than the rest.
However, I today ran into a situation where I experienced exactly such a long
tail using DistCpV2 and DynamicInputFormat. And when I tried to alleviate the
problem by overriding the number of mappers and the split ratio used by the
DynamicInputFormat, I was prevented from doing so by the hard-coded limit set
in the code by the MAX_CHUNKS_TOLERABLE constant. (Currently set to 400.)
This constant is actually set quite low for production use. (See a description
of my use case below.) And although MAPREDUCE-2765 states that this is an
"overridable maximum", when reading through the code there does not actually
appear to be any mechanism available to override it.
This should be changed. It should be possible to expand the maximum # of
chunks beyond this arbitrary limit.
For example, here is the situation I ran into today:
I ran a distcpv2 job on a cluster with 8 machines containing 128 map slots.
The job consisted of copying ~2800 files from HDFS to Amazon S3. I overrode
the number of mappers for the job from the default of 20 to 128, so as to more
properly parallelize the copy across the cluster. The number of chunk files
created was calculated as 241, and mapred.num.entries.per.chunk was calculated
as 12.
As the job ran on, it reached a point where there were only 4 remaining map
tasks, which had each been running for over 2 hours. The reason for this was
that each of the 12 files that those mappers were copying were quite large
(several hundred megabytes in size) and took ~20 minutes each. However, during
this time, all the other 124 mappers sat idle.
In theory I should be able to alleviate this problem with DynamicInputFormat.
If I were able to, say, quadruple the number chunk files created, that would
have made each chunk contain only 3 files, and these large files would have
gotten distributed better around the cluster and copied in parallel.
However, when I tried to do that - by overriding mapred.listing.split.ratio to,
say, 10 - DynamicInputFormat responded with an exception ("Too many chunks
created with splitRatio:10, numMaps:128. Reduce numMaps or decrease split-ratio
to proceed.") - presumably because I exceeded the MAX_CHUNKS_TOLERABLE value of
400.
Is there any particular logic behind this MAX_CHUNKS_TOLERABLE limit? I can't
personally see any.
If this limit has no particular logic behind it, then it should be overridable
- or even better: removed altogether. After all, I'm not sure I see any need
for it. Even if numMaps * splitRatio resulted in an extraordinarily large
number, if the code were modified so that the number of chunks got calculated
as Math.min( numMaps * splitRatio, numFiles), then there would be no need for
MAX_CHUNKS_TOLERABLE. In this worst-case scenario where the product of numMaps
and splitRatio is large, capping the number of chunks at the number of files
(numberOfChunks = numberOfFiles) would result in 1 file per chunk - the maximum
parallelization possible. That may not be the best-tuned solution for some
users, but I would think that it should be left up to the user to deal with the
potential consequence of not having tuned their job properly. Certainly that
would be better than having an arbitrary hard-coded limit that *prevents*
proper parallelization when dealing with large files and/or large numbers of
mappers.
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